Lewis Hamilton is poised to start the Belgian Grand Prix from the very back of the field after serving a mammoth 30-place grid penalty following further changes to his engine.

The reigning world champion, who leads his Mercedes team-mate Nico Rosberg by 19 points in this year's title race, was already staring at a 15-place grid drop after his team confirmed he has taken on a new turbo and MGU-H, exceeding the five he is allowed before a penalty is enforced.

The penalty has been looming over the British driver after he encountered a number of engine problems in the opening rounds of the season.

Hamilton's Mercedes team had been weighing up whether to take the grid drop here at Spa or at next Sunday's Italian Grand Prix, with both circuits expected to offer him the best chance of fighting back through the field.

But after confirming the penalty on Thursday, replacement components were fitted on to Hamilton's engine in opening practice with additional parts added in the afternoon session, raising the running total to 30.

Under the sport's complicated rules, he could yet be given an even greater penalty should Mercedes opt to fit further new components to Hamilton's engine before qualifying here on Saturday.

But with only 22 competitors, the size of Hamilton's penalty will actually matter little with the British driver simply relegated to the back of the field. He will be joined there by McLaren's Fernando Alonso who is also set to incur a 30-place drop for using more engine parts than is permitted.

Hamilton's penalty means he may only choose to run once in qualifying with his position at the back of the field now guaranteed. That would come as a blow to the thousands of British fans who have headed to Belgium to support the world champion.

No driver in the history of the sport has ever won from last on the grid. John Watson, the Belfast-born former McLaren driver, holds the record of winning from the lowest start slot after lining up in 22nd on the grid of 26 runners before winning the 1983 United States Grand Prix at Long Beach.

But Hamilton, speaking on Thursday, did not rule out claiming what would be a remarkable victory on Sunday.

"In terms of winning that is the goal, but it is going to be very, very hard," said Hamilton, who is just one win shy of 50 career victories.

"I will do everything I can to minimise the impact that the penalty will have, and beyond that I plan to continue with the momentum I had before the break."